In a TET system, a power supply is electrically connected to a primary coil that is external to a physical boundary, such as the skin of the human body. A secondary coil is provided on the other side of the boundary, such as internal to the body. With a subcutaneous device, both the primary and secondary coils are generally placed proximate to the outer and inner layers of the skin. Energy is transferred from the primary coil to the secondary coil in the form of an alternating magnetic field. The secondary coil converts the transferred energy in the AC magnetic field to electrical power for the implant device, which acts as a load on the secondary coil.
In a TET system, the primary and secondary coils are placed on separate sides of the boundary or skin. This separation typically results in variations in the relative distance and spatial orientation between the coils. Variations in the spacing can cause changes in the AC magnetic field strength reaching the secondary coil, in turn causing power fluctuations and surges in the implant device. Implant devices, such as those used in medical applications, usually rely upon a microcontroller to perform various functions. These microcontrollers require a consistent, reliable power source. Variations in the supplied power, such as sudden changes in voltage or current levels, may cause the device to perform erratically or fail to function at all. Accordingly, one issue associated with conventional TET systems is that the physical displacement of either the primary or secondary coils from an optimum coupling position may cause an unacceptable effect on the output power supplied to the implanted device.
As an example of an implantable device that may benefit from use of TET is an artificial sphincter, in particular an adjustable gastric band that contains a hollow elastomeric balloon with fixed end points encircling a patient's stomach just inferior to the esophago-gastric junction. These balloons can expand and contract through the introduction of saline solution into the balloon. In generally known adjustable gastric bands, this saline solution must be injected into a subcutaneous port with a syringe needle to reach the port located below the skin surface. The port communicates hydraulically with the band via a catheter. While effective, it is desirable to avoid having to adjust the fluid volume with a syringe needle since an increased risk of infection may result, as well as inconvenience and discomfort to the patient.
To that end, in the below-referenced co-pending applications, an implanted infuser device regulates the flow of saline without requiring injection into the subcutaneous port. This system instead transfers AC magnetic flux energy from an external primary coil to a secondary coil that powers the pump in the implant connected to the gastric band within the abdomen.
Although such TET powering of an implant, such as to recharge batteries, is a generally known procedure, using TET for an artificial sphincter system, such as an adjustable gastric band, presents a number of challenges. Adjustable gastric bands are most beneficial to patients that are morbidly obese. Providing a secure location to subcutaneously attach an implant that presents a reduced incident of discomfort often means that the implant is under a thick layer of skin and adipose tissue. A major challenge in using TET thus is transferring magnetic energy between the primary and secondary coils through this thick layer of tissue, which thus reduces the effective amount of power transferred to the implant.
It is also generally known to include a magnetic shield across an external side of a primary coil used in TET powering of an artificial heart, such as described in U.S. Pat. No. 6,389,318. Such magnetic shields are generally a flat disk that overlays the top and sides of the primary coil for the purpose of shielding from other conductors in the external environment. Perforations are included for ventilation since such primary coils are continually positioned on the patient. To be conformal, a preferred material is silicon impregnated with ferrite powder so that its low magnetic loss serves as a back plane that reflects magnetic energy from the primary coil. While providing advantages for external sources of electromagnetic interference, such shields are not believed to substantially assist in directing the magnetic flux to the secondary coil of an implanted medical device.
While the shield described in U.S. Pat. No. 6,389,318 provided some shaping of the magnetic flux from the primary coil, one undesirable characteristic thereof was that the magnetic flux was flattened, providing less efficient power coupling to deeply embedded implantable devices. In the application described for artificial hearts, the secondary coil was near to the surface of the patient's skin and thus this apparently did not pose a problem.
In U.S. Pat. No. 5,715,837, enhancing the effectiveness of TET was addressed by increasing the magnetic permeability of the flux path through the dermis of the patient by implanting soft iron pellets therein. It would be undesirable to implant metal pellets for a number of reasons. First, in the morbidly obese patient, it may require a significant amount of pellets to seed the flux path. Second, the patient may object to this permanent implantation. Third, being ferrous objects, tissue damage or discomfort may result if the patient were in the presence of a strong magnetic field typical of a Magnetic Resonance Imaging (MRI) machine. Fourth, these ferrous objects would create artifacts that would hamper diagnostic imaging such as MRI and CT. Fifth, the chemical or physical properties of these pellets may have a deleterious effect on the dermis.
It is further inconvenient to shape the magnetic flux as described in U.S. Pat. No. 5,715,837 with opposing horseshoe shaped ferrite cores insofar as it is desirable to eliminate such mass from an implanted device to make it smaller. In addition, it is further desirable to eliminate materials that respond to strong magnetic fields, as mentioned with regard to soft iron pellets or a partially exposed, implanted ferrite core.
In U.S. Pat. No. 5,279,292, a charging system for an implantable hearing aid or tinnitus masker included a receiving coil that is implanted under the skin in mastoid. The receiving coil included a ferrite core that projected outward through the skin. Thus, a transmitting coil is placed over the exposed end of the ferrite core, mechanically aligning the primary coil and enhancing magnet coupling to the receiving core. Due to the relatively small amount of power transferred, the ferrite core is described as being small and unobtrusive and being hidden behind the external ear. However, it is undesirable to have an exposed implant that tends to allow infections. In addition, use of a transformer instead of TET as in this application also makes the implant not compatible with MRI machines.
Consequently, a significant need exists for enhancing TET power transfer from a primary coil through the dermis of a patient to an implanted device that contains a secondary coil.